Heat Index Calculator: Know the Real Temperature Before Your Next Hike

Calculate the heat index using air temperature and humidity. The heat index shows how hot it actually feels to your body accounting for humidity levels.

Enter Conditions




How to Use This Calculator

Enter the current air temperature in Fahrenheit or Celsius and the relative humidity percentage. If you are in direct sunlight with little wind, check the sun exposure box. Click Calculate to see the heat index and safety recommendations. The calculator uses the Rothfusz regression equation with NWS adjustments, the same formula used by the National Weather Service.

Understanding Heat Index

The heat index was developed to provide a more accurate measure of how hot it truly feels when humidity is factored into the air temperature. Your body cools itself primarily through perspiration. When sweat evaporates from your skin, it carries heat away from your body. However, when humidity is high, the air is already saturated with moisture, which slows or prevents sweat evaporation. This forces your body temperature to rise, potentially leading to dangerous heat-related illnesses.

The Rothfusz regression equation, adopted by the NWS, uses multiple regression analysis to approximate the heat index. The formula includes adjustments for low humidity at moderate temperatures and high humidity at lower heat index temperatures. Direct sunlight can add up to 15 degrees Fahrenheit to the heat index because radiant heat from the sun adds thermal load to your body beyond what air temperature alone produces.

Safety Tips for Hot Weather Adventures

  • Hydrate Before You Feel Thirsty: Drink at least 16-24 ounces of water per hour during activity.
  • Time Your Activities: Schedule strenuous hiking for early morning or late afternoon. Avoid peak heat between 10 AM and 4 PM.
  • Wear Light, Loose Clothing: Choose moisture-wicking, light-colored fabrics that allow air circulation.
  • Know the Warning Signs: Learn to recognize heat cramps, heat exhaustion, and heat stroke symptoms.
  • Use Electrolytes: Water alone is not enough. Add electrolyte tablets or sports drinks to replace sodium and potassium.
  • Plan Your Route: Know where shade and water sources are along your trail. Carry more water than you think you need.
  • Buddy System: Never hike alone in extreme heat. A partner can recognize heat illness symptoms you might miss.

Frequently Asked Questions

What is the heat index?
The heat index, also known as the apparent temperature, combines air temperature and relative humidity to determine what the temperature actually feels like to the human body. High humidity prevents sweat from evaporating efficiently, reducing your body’s ability to cool itself.
At what heat index should you stop outdoor activities?
The NWS recommends extreme caution at heat index values of 90-103°F, and considers 103-124°F dangerous. At a heat index above 125°F, conditions are classified as Extreme Danger with a high likelihood of heat stroke.
How does humidity affect how hot it feels?
Humidity prevents sweat from evaporating off your skin. At 90°F with 40% humidity, the heat index is about 91°F. But at 90°F with 90% humidity, the heat index jumps to approximately 122°F.
What are the signs of heat exhaustion vs heat stroke?
Heat exhaustion: heavy sweating, cold and clammy skin, fast weak pulse, nausea, fatigue. Heat stroke: body temperature above 103°F, hot red dry skin, fast strong pulse, confusion, loss of consciousness. Call 911 immediately for heat stroke.
Does the heat index account for direct sunlight?
The standard heat index is calculated for shady conditions. Direct sunlight can increase the heat index by up to 15°F. This calculator includes an optional sun exposure adjustment for more accurate readings.

About the Author

Mark Spencer is an adventure travel expert and wilderness safety instructor with over 15 years of experience exploring remote destinations across all 50 US states. As the founder of Adventure Hidden, Mark is certified in Wilderness First Aid and is a Leave No Trace Master Educator. He has led hundreds of guided expeditions through challenging terrain and is dedicated to helping adventurers stay safe.